Process for regulating the tool inner pressure curve of a cyclically working machine

ABSTRACT

For a cycle of a cyclically operating machine, the internal tool pressure is detected during the cycle as a function of the time and/or as a function of the path traversed by the conveying means. The internal tool pressure course is then differentiated as a function of the time or of the path. It is then checked whether, by way of the previously determined derivatives, there are deviations from a monotonic course. If this is the case, the holding pressure time or the change-over moment are changed until the internal tool pressure curve has a monotonic course.

DESCRIPTION REFERENCE TO RELATED APPLICATIONS

The present application claims the priority of German Patent Application195 36 566.6, filed on Oct. 2, 1995, the disclosure contents of which isherewith also explicitly made the subject matter of the presentapplication.

TECHNICAL FIELD

The invention relates to a method of controlling an internal toolpressure on a cyclically operating machine.

The method can preferably be used in the plastics-processing andmetal-working industries, for example, in conjunction with plasticsinjection molding machines, blow molding machines, die-casting machines,aluminum die-casting machines, but also in conjunction with presses orwelding apparatuses provided that cyclically operating processes arecarried out. In so far as process curve courses are discussed in thefollowing text, this covers not only courses that can be detecteddirectly but also those process curve courses that can be calculatedfrom the detected courses such as, for example, derivatives, integralsand the like.

STATE OF THE TECHNOLOGY

EP-A 233 548 discloses a method to detect the process curve courses ofthe hydraulic pressure in a cycle-synchronous manner. In order to attaina change-over point control between compression and holding pressurephase, specific criteria are established that are checked by way of thehydraulic pressure course. It is checked, for example, that the pressurefor the change-over to holding pressure is reached only once, that thepressure course does not contain any overshooting and no pressure peak;but all of these checks are not carried out by way of the internal toolpressure. The drawback of this procedure which focuses on the hydraulicpressure, however, is that this pressure is setting-dependent andmachine-dependent and is therefore difficult to evaluate. A control ofthe holding pressure time including corresponding criteria cannot befound in this disclosure.

DE-A 30 21 978 describes a method of controlling the internal toolpressure course on a cyclically operating machine, with the internaltool pressure course as well as the hydraulic pressure course beingmonitored as decision criteria for the determination of the change-overpoint. To accomplish a pressure shock-free change-over from theinjection phase to the holding pressure phase and from the latter to thesetting phase, the hydraulic pressure is controlled as soon as theactual value of the tool pressure reaches the desired starting value ofthe holding pressure phase, and an actual value of the pressure ispredetermined at the transfer point as desired starting value of thesetting phase. While this permits the transition of the internal toolpressure from one into another, it does not yet ensure a harmoniccontinuous pressure course.

SUMMARY OF THE INVENTION

Starting from this state of the technology, it is an object of thepresent invention to optimize and monitor the cycle on machines of thistype so that high-quality parts are produced.

The above and other objects are accomplished according to the inventionby the provision of a method of controlling a course of an internal toolpressure on a cyclically operating machine, comprising the steps of:

a) filling of a mold cavity with a conveyable material by a conveyingmeans,

b) applying a pressure to compress the conveyed material while thefilling of the mold cavity continues,

c) applying a holding pressure during a molding of a product in the moldcavity,

d) reducing the pressure and removing the product,

e) detecting the course of the internal tool pressure (Pwi) at least asa function of time (t), which course occurs as a result of the pressurein steps a) to d),

f) differentiating the course of the internal tool pressure (Pwi) atleast twice as a function of time (t) to determine a second orderderivative,

g) by way of the derivative determined in step (f), checking the courseof the internal tool pressure (Pwi) for presence of an extreme value(max(dPwi/dt)²) of the second order derivative at least in a transitionregion between the regions of the internal tool pressure coursesresulting from steps b) to c),

h) changing a change-over time (t_(um)) of a transition between steps b)and c) when an extreme value of the second order derivative occurs instep g), and

i) repeating steps a) to h) until the second order derivative no longershow an extreme value in the transition region.

According to yet another embodiment of the invention there is provided amethod of controlling a course of an internal tool pressure on acyclically operating machine, comprising the steps of:

a) filling of a mold cavity with a conveyable material by a conveyingmeans,

b) applying a pressure to compress the conveyed material while thefilling of the mold cavity continues,

c) applying a holding pressure during a molding of a product in the moldcavity,

d) reducing the pressure and removing the product,

e) detecting the course of the internal tool pressure (Pwi) at least asa function of a path traversed by the conveying means, which courseoccurs as a result of the pressure in steps a) to d),

f) differentiating the course of the internal tool pressure (Pwi) atleast twice as a function of the path to determine a second orderderivative,

g) by way of the derivative determined in step (f), checking the courseof the internal tool pressure (Pwi) for the presence of an extreme value(Max(dPwi/ds)²) of the second order derivative at least in a transitionregion between the internal tool pressure courses resulting from stepsb) to c),

h) changing a change-over point of the path traversed by the conveyingmeans up to a transition between steps b) and c) when an extreme valueof the second order derivative occurs, and

i) repeating steps a) to h) until the second order derivative no longershows an extreme value in the transition region.

According to a still further embodiment of the invention there isprovided a method of controlling a course of an internal tool pressureon a cyclically operating machine, comprising the steps of:

a) filling of a mold cavity with a conveyable material by a conveyingmeans,

b) applying a pressure to compress the conveyed material while thefilling of the mold cavity continues,

c) applying a holding pressure during a molding of the product in themold cavity,

d) reducing the pressure and removing the product,

e) detecting the course of the internal tool pressure (Pwi) at least asa function of time (t), which course occurs as a result of the pressurein steps a) to d),

f) differentiating the course of the internal tool pressure (Pwi) atleast one time as a function of the time (t),

g) by way of the derivative determined in step (f), checking the courseof the internal tool pressure (Pwi) for presence of a constant slope atleast in regions of the internal tool pressure courses resulting fromsteps c) to d),

h) when a non-constant descending course occurs in step g), changing aholding pressure time (t_(n)) up to a transition between steps c) andd),

i) repeating steps a) to h) until a constantly descending course appearsin step f).

The internal tool pressure course is checked with the assistance ofdifferential calculus. For this purpose, this internal tool pressurecourse is recorded during the different phases of a cycle. If the goalis an optimum change-over point between injection phase and holdingpressure phase, the curve is checked for a monotonic slope (i.e. lineargradient). If there is no monotonic course, the time periods up to orthe path up to the change-over point are changed until an optimumchange-over point is determined. If the goal is an optimum holdingpressure time up to the sealing of the sprue, the curve is checked for aconstant drop in a region of the end of the holding pressure, that is,to ensure that the curve does not have a kink difference or slopedifference between adjacent points. The holding pressure time can alsobe optimized. In this manner, the cycle time can be optimized withoutloss of quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 are digrams showing the internal tool pressure course and thehydraulic pressure course for a sufficient holding pressure time and aholding pressure time that is too short and for a semi-crystallinematerial,

FIGS. 3, 4 are diagrams showing curves according to FIGS. 1, 2 When anamorphous material is used,

FIGS. 5, 6 are digrams which show the internal tool pressure course, thescrew stroke as well as the course of the derivative of the second orderwith regard to the internal tool pressure as a function of the time fordetermining a change-over point for an optimum change-over point and apremature change-over,

FIG. 7 is a flow chart showing the method sequence in the form of ablock diagram for determining an optimum holding pressure time,

FIG. 8 is a flow chart showing the method sequence by way of a blockdiagram for determining an optimum change-over moment,

FIG. 9 is a schematic and partial block digram showing the elementsprovided on a plastics injection molding machine,

FIG. 10 is a flow chart showing a method sequence with regard to aquality control,

FIG. 11 is a digram showing, according to FIG. 5, a course of thederivative of the internal tool pressure as a function of the path.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is now described by way of example with reference to theattached drawings in greater detail. The embodiments, however, aremerely examples which are not intended to limit the concept of theinvention to a specific physical arrangement.

The method and the arrangement required therefor are explained below byway of an injection molding machine, preferably an injection moldingmachine for the processing of plastic compounds such as plasticmaterials or powdered compounds. But, as was mentioned at the outset,the method can also be used without any problems in other fields inwhich cyclically occurring processes are used for the production(molding) or processing (forming) of products.

The method serves to control the internal tool pressure course on acyclically operating machine. It is based on the fact that it turns outthe internal tool pressure Pwi is best suited to display discrepancieswhen it comes to making optimum high-quality products. With thesemachines, the production sequence of the products is approximately likethe sequence described in detail below for an injection molding machine.

Referring to FIG. 9, an injection means 31 of an injection molding unitsuch as, for example, a conveying screw or an injection plunger injectmaterial into a mold cavity 32 of an injection mold 34 in a mold closingunit 33. The arrangement is provided with sensor means such as, forexample, an internal tool pressure sensor 11 and a temperature sensor 12in the region of the injection molding unit. When the conveying means ismoved, the mold cavity is first filled gradually; during this process,the internal tool pressure at first starts rising very graduallyaccording to FIG. 1. If the internal tool pressure sensor is notarranged in the proximity of a sprue, a delayed start of the processcurve course can also occur, as is illustrated in FIGS. 5 and 6. But itis preferred that the internal tool pressure sensor 11 is arranged inthe proximity of the sprue because otherwise problems might arise withregard to the detection of the internal tool pressure Pwi, at least inthin-walled parts. The mold cavity is now filled at the level of thekink Un in the internal tool pressure curve Pwi. The conveying means,however, continues to operate to press still more material into the moldcavity during a compression phase. At this moment, the conveying means31 reaches a point at which a further forward movement in the directionof the mold cavity is no longer possible, so that a path or velocitycontrol can no longer take place. The change-over to a pressure controltakes place at this change-over time t_(um) and a holding pressurecontrol begins, which corresponds approximately with region C shown inFIG. 5.

The moment of the change-over set on a machine determines, e. g., when achange-over takes place from the velocity-controlled injection to thepressure-controlled holding pressure region. The change-over pointshould correspond, if possible, with the volumetric filling of the moldcavity 32 because the mold cavity is intended to be filled at a definedvelocity, but the shrinkage occurring in the holding pressure phase mustbe compensated for by a predetermined pressure profile. However, at thechange-over point, pressure fluctuations and pressure peaks must beavoided, if possible, so that a continuous and harmonic transitionoccurs between injection phase and holding pressure phase and that thequality of the molding cannot be impaired by negative influences.

Then there also occurs a maximum Max(Pwi) of the internal tool pressurePwi in region C of the holding pressure. Once the maximum is exceeded,the pressure gradually drops during the cooling of the molding. Here, itis now decisive to determine when the molding is sealed in the tool.This moment must be equated with an optimum holding pressure time. Ifthe holding pressure time is set to be too long, it may become themagnitude which determines the cycle time and may therewith reduce theproductivity and efficiency of the process. But if it is too short, themolding is not sealed and melt flows out of the tool. This loss of massimpairs the quality of the moldings and their reproducibility.

FIGS. 1 and 2 differ from FIGS. 3 and 4 in that an amorphous material isprocessed in FIGS. 3 and 4, whereas semi-crystalline material isprocessed in FIGS. 1 and 2. Since the forming of the product takes arather long time in the case of the amorphous material, the internaltool pressure Pwi must also be maintained for a longer time period.

The hydraulic pressure Phy is also plotted in FIGS. 1-4. It isunderstood that this hydraulic pressure can be used on a hydraulicmachine as setting magnitude for the internal tool pressure, but that asimilar picture emerges when other drives such as, e.g.,electromechanical drives are used, in which cases the hydraulic pressurePhy is then replaced by a force course. The hydraulic pressure is highin the beginning of the filling phase in order to accomplish a fillingof the mold cavity 31 as quickly as possible. In the holding pressurephase, the hydraulic pressure then is at a level P1 from which it dropsto a level P2 at the end of the holding pressure time.

Referring additionally to FIG. 7, the process curve courses discussed sofar, but also those illustrated in FIGS. 5 and 6, are now detectedduring the implementation of an injection cycle according to step S1 atleast as a function of the time t and/or as a function of the path straversed by the conveying means 31 according to step S2. With means ofdifferential calculus, the process curve courses, but substantially theinternal tool pressure courses, are differentiated as a function of thetime t or of the path s according to step S3. Then a check takes placein step S4 as to whether a monotonic or constant slope can be determinedby way of the derivatives of the first, second and third order that weredetermined. If the optimum has not yet been reached, it is checked instep S5 in FIG. 7, which represents an example of a controller for theholding pressure time, whether a constant slope, that is, e.g., a kink,is not present, as is illustrated, for example, in FIGS. 2 and 4. In theabsence of a constant ascending slope, more precisely of a constantdrop, the holding pressure time t_(n) is increased in the example ofFIG. 7 to be

    t.sub.n +1=t.sub.n +δt.

This is intended to accomplish that the drop of the internal toolpressure, which results from a premature pressure reduction of theholding pressure as a consequence of escaping material, is preventedduring the next injection cycle (step S6). The next injection cycle isthen carried out with the changed holding pressure time t_(n) and, if anoptimum already emerges, only steps S1 to S4 are carried out after that.

But a constantly dropping course in this region is also brought about ifthe holding pressure time t_(n) is selected to be too long. For thisreason, a control can be provided to the extent that a further change ofthe holding pressure time t_(n) is carried out, even if it turns out instep S5 that a constant slope is present. In this case, the holdingpressure time t_(n) is now changed according to step S7 to be

    t.sub.n +1=t.sub.n -δt.

This means that the holding pressure time is reduced and the nextinjection cycle takes place. This reduction of the holding pressure timetakes place until a constant slope again does not take place. If theholding pressure time is now increased slightly, an optimum holdingpressure time t_(n) is brought about.

This control of the holding pressure time t_(n) can first be carried outin that a plurality of points along the internal tool pressure course ischecked; and in the absence of a constant slope in a closed loop controlsystem, the holding pressure time t_(n) is controlled to a value atwhich a constant slope just no longer occurs. For this check, theinternal tool pressure course Pwi is differentiated once as a functionof the time t, and the value S1 of the slope in a region B1 upstream ofthe point t_(n) and the value S2 in a region B2 downstream of the pointare determined. If the result of the division of the value S1 by thevalue S2 is a value of approximately 1, then a continuity exists.

But the search algorithm for finding a kink Un can be accelerated if thechecking takes place, e.g., starting from the point at which the digitalsignal "end of holding pressure" 10 is generated in FIG. 2. As analternative to the digital signal "end of holding pressure", thenegative edge in the hydraulic pressure Phy at the end of the holdingpressure can also be used. At the end of the holding pressure, thehydraulic pressure Phy drops from a high pressure level P1 to a lowlevel P2. The drawback of this procedure is that both pressure levelsare setting-dependent and machine-dependent and it is therefore moredifficult to evaluate them. Alternatively, the digital signal "end ofholding pressure" can be determined through the digital signal "opentool" and the value of the machine setting magnitude "remaining coolingtime" because the moment at the end of the holding pressure correspondsto the moment "open tool" minus the remaining cooling time. The holdingpressure time can thus be controlled according to the controllerequation:

    Δt.sub.n =-((S1/S2)-desired value)×amplification factor.

It is also possible to control the change-over point with a similaralgorithm; therefore, the following makes reference to FIGS. 5, 6 and 8.In FIGS. 5 and 6, the internal tool pressure Pwi, on the one hand, andits derivative of the second order, on the other hand, are plotted ineach case over time. Since the derivative of the internal tool pressurePwi can also take place as a function of the path traversed by theconveying means 31, FIG. 11 thus shows a representation that can becompared with FIG. 5. In the following, the otherwise identical controlalgorithm is illustrated by way of the time-dependent differentiation.

FIG. 5 shows a correct change-over wherein a monotonic or strictlymonotonicly ascending course of the internal tool pressure curve isbrought about in the region of the change-over point t_(um). Thesituation is different in FIG. 6 where a premature change-over takesplace which consequently results in a pressure collapse which isreflected in extreme values, that is, maximum and minimum values of thederivatives. If the change-over point is not correct, a marked localmaximum as well as an associated local minimum of the second derivativeof the internal pressure usually appear. The criterion is whether thederivative of the second order has an extreme value. Controlling takesplace for a state of the change-over which is not quite perfect. Theerror of a slightly premature change-over is necessary since, otherwise,there is no criterion to determine when the change-over occurs too late.The use of the derivative of the second order of the internal toolpressure leads to the fact that a very small error during thechange-over suffices to arrive at only a detectable slight delay but notat the stopping of the screw. If the same were attempted with the firstderivative of the internal tool pressure, only a zero passage couldserve as a characteristic value. But this presupposes a pressurecollapse, that is, not only a temporary deceleration of the pressurerise. But this is usually not without effect on the molding.

It was already explained in the beginning why the derivative of thesecond order is advantageous compared to the first derivative. Ifpreviously an optimum was not achieved, which can only be determined byway of a comparison after at least two injection cycles, it is examinedin step S8 (see FIG. 8) whether

    (dPwi/dt).sup.2 ˜0.

If this is not the case, the change-over moment t_(um) is increased byδt in accordance with step S10. If, however, a value in a magnitude ofaround zero was already present, the change-over moment can now bereduced by δt, as was already explained above for the holding pressuretime controller, so as to thus determine the moment at which an optimumchange-over time is present; this not only contributes to producing ahigh-quality molding but it also helps to reduce the cycle time. It isthe goal to accomplish a control to the extent that the change-over timet_(um) is disposed where an extreme value of the derivative of thesecond order just no longer occurs.

Here, there are also criteria which accelerate the search algorithm, sothat it is no longer necessary to visit and examine each individualpoint of the internal tool pressure course. Because the maximum orminimum, that is, the extreme value Max(dPwi/dt)², is searched forbetween the change-over time t_(um) set on the machine and the timet_(m) at which the first maximum Max (Pwi) of the internal tool pressureoccurs in the holding pressure phase C. Usually, a threshold value of ascrew position or a fixed moment is indicated as a criterion for thesearch, from which the search can start. A threshold value of aninternal tool pressure or of a hydraulic pressure can also be acriterion. This results in a controller equation either as a function ofthe time t or of the screw position such as:

    s=(Min)Max((dPwi/dt).sup.2 -desired value)×amplification factor×S.factor.

The S.factor is the scaling factor and is intended for the adjustment tothe value scale which is available in the machine as a setting magnitudesuch as, e.g., screw path, injection volume or time. The value s canassume very large values so that it cannot be utilized directly assetting magnitude. Of the value that was determined only the sign isused and multiplied by a fixedly predetermined small adjustment unit soas to obtain a meaningful adjustment.

Holding time controller and change-over point controller can be part ofa multimagnitude controller for controlling the quality of the featuresthat are produced, as is explained in the older German patentapplication P 44 34 653.0 and the associated PCT application PCT/DE95/01345 filed on Sep. 28, 1995. The disclosure content of theseapplications is herewith also explicitly made the disclosure content ofthe present application. A short version thereof ensues from FIGS. 9 and10.

During each process, a plurality of process parameters is determined viasensor means such as, e.g., an internal tool pressure sensor 11 and atemperature sensor 12. But this determination first takes place during atest phase T during which the machine setting magnitudes MG aresystematically changed. For each change, the quality features QM of theproducts that are made as well as the machine setting magnitudes (stepsS1', S2', S3') which are set during the process are detected. Processcurve courses PKV are detected during a process via means 13. Thequality features QM can additionally be entered via input means 14,which quality features can be determined by conventional methods butalso directly by the machine.

In step S4', an usually automatic breakdown of the process curve coursesinto process phases PPH takes place which are determined by the machineitself by way of the signal input. Here, different signals of differentprocess curve courses can be associated with one another to determinemeaningful process phases PPH. Within these process phases, the processidentifier numbers PKZ are then formed which identify the process phases(step S5'). A connection between machine setting magnitude and qualityfeature QM can then be determined via these process identifier numbers.Corresponding equations are shown in step S6'. These relationships aredetermined through the means 15. Usually, neither the quality featuresnor the machine setting magnitudes are linearly dependent from oneanother. In step S10', desired values QM_(desired), MG_(desired) arepredetermined for the quality features QM and the machine settingmagnitudes MG. During the production phase P, the same steps are carriedout which already took place during the test phase. Series process curvecourses SPKV are determined (step S7'), these are broken down intoseries process phases SPPH (step S8') and are identified by seriesprocess identifier numbers SPKZ (step S9'). By way of these seriesprocess identifier numbers, a monitoring and control of the quality ofthe parts to be produced can be carried out in step S12'.

For this purpose, the relationship determined in the test phase wasstored in storage means 16. Then the machine setting magnitudes can beinfluenced, for example, via the setting arrangement 17, 18. If adifference value QM, PKZ, MG is obtained in a comparator 21, thisdifference value can be evaluated by controller 19 to the extent that anadjustment control takes place.

FIG. 11 illustrates the determined internal tool pressure as well as itssecond differential derivative (d Pwi/ds)² as a function of the path sof the conveying means, plotted over the time. In an injection cycle onan injection molding machine, the time domain covers approximately arange of 0.3 sec prior to the change-over to approximately 0.7 sec afterthe change-over. A correct change-over point is shown at which thesecond derivative in the time period considered is approximately aroundzero. If, however, the change-over occurs too late, the secondderivative experiences a marked minimum. If, on the other hand, thechange-over occurs too early, a difference cannot be found compared tothe case of the correct change-over. For this reason, the control takesplace such that only a slight decrease of the pressure rise occursaccording to a small extreme value of the second derivative, so that achange-over point is accomplished which is as optimal as possible.

It is understood that this description can be subjected to a greatvariety of modifications, changes and adjustments which are in the rangeof equivalents with respect to the attached claims.

We claim:
 1. A method of controlling a course of an internal toolpressure on a cyclically operating machine, comprising the steps of:a)filling of a mold cavity with a conveyable material by a conveyingmeans, b) applying a pressure to compress the conveyed material whilethe filling of the mold cavity continues, c) applying a holding pressureduring a molding of a product in the mold cavity, d) reducing thepressure and removing the product, e) detecting the course of theinternal tool pressure (Pwi) at least as a function of time (t), whichcourse occurs as a result of the pressure in steps a) to d), f)differentiating the course of the internal tool pressure (Pwi) at leasttwice as a function of time (t) to determine a second order derivative,g) by way of the derivative determined in step (f), checking the courseof the internal tool pressure (Pwi) for presence of an extreme value(max(dPwi/dt)²) of the second order derivative at least in a transitionregion between regions of the internal tool pressure courses resultingfrom steps b) to c), h) changing a change-over time (t_(um)) of atransition between steps b) and c) when an extreme value of the secondorder derivative occurs in step g), and i) repeating steps a) to h)until the second order derivative no longer shows an extreme value inthe transition region.
 2. The method according to claim 1, including:j)advancing the change-over time (t_(um)) when the second order derivativeis near zero in step g), k) repeating steps a) to g) until the secondorder derivative has an extreme value in a region of the change-overtime, and performing steps h) and i) by delaying the change-over time(t_(um)) slightly until an extreme value just no longer occurs.
 3. Themethod according to claim 1, including continuously detecting the courseof the internal tool pressure (Pwi) and controlling the change-over timein a closed control loop to a value so that the extreme value just nolonger occurs.
 4. The method according to claim 1, including determiningthe extreme value (Max(dPwi/dt)²) between the change-over time (t_(um))set on the machine and a time (t_(m)) at which a first maximum(Max(Pwi)) of the internal tool pressure appears while the holdingpressure is applied in step c).
 5. The method according to claim 1,further including providing at least one of the following criteria as achange-over criterion set on the machine between step b) and c):1)reaching a predetermined path of the conveying means, 2) reaching apredetermined moment, 3) reaching a predetermined internal toolpressure, and 4) reaching a predetermined hydraulic pressure, on a basisof which the method steps are carried out.
 6. A method of controlling acourse of an internal tool pressure on a cyclically operating machine,comprising the steps of:a) filling of a mold cavity with a conveyablematerial by a conveying means, b) applying a pressure to compress theconveyed material while the filling of the mold cavity continues, c)applying a holding pressure during a molding of a product in the moldcavity, d) reducing the pressure and removing the product, e) detectingthe course of the internal tool pressure (Pwi) at least as a function ofa path traversed by the conveying means, which course occurs as a resultof the pressure in steps a) to d), f) differentiating the course of theinternal tool pressure (Pwi) at least twice as a function of the path todetermine a second order derivative, g) by way of the derivativedetermined in step (f), checking the course of the internal toolpressure (Pwi) for the presence of an extreme value (Max(dPwi/ds)²) ofthe second order derivative at least in a transition region between theinternal tool pressure courses resulting from steps b) to c), h)changing a change-over point of the path traversed by the conveyingmeans up to a transition between steps b) and c) when an extreme valueof the second order derivative occurs, and i) repeating steps a) to h)until the second order derivative no longer shows an extreme value inthe transition region.
 7. The method according to claim 6 including:j)shortening the path traversed by the conveying means up to thechange-over point when the second order derivative is almost zero instep g), k) repeating steps a) to g) until the second order derivativehas an extreme value in a region of the change-over point, andperforming steps h) and i) by extending slightly the path traversed bythe conveying means until an extreme value just no longer occurs.
 8. Themethod according to claim 6 including continuously detecting the courseof the internal tool pressure (Pwi) and controlling the path up to thechange-over point in a closed control loop to a value so that a maximumjust no longer occurs.
 9. The method according to claim 6, includingproviding at least one of the following criteria as a change-overcriterion set on the machine:1) reaching a predetermined path of theconveying means, 2) reaching a predetermined moment, 3) reaching apredetermined internal tool pressure, and 4) reaching a predeterminedhydraulic pressure, on a basis of which the method steps are carriedout.
 10. A method of controlling a course of an internal tool pressureon a cyclically operating machine, comprising the steps of:a) filling ofa mold cavity with a conveyable material by a conveying means, b)applying a pressure to compress the conveyed material while the fillingof the mold cavity continues, c) applying a holding pressure during amolding of the product in the mold cavity, d) reducing the pressure andremoving the product, e) detecting the course of the internal toolpressure (Pwi) at least as a function of time (t), which course occursas a result of the pressure in steps a) to d), f) differentiating thecourse of the internal tool pressure (Pwi) at least one time as afunction of the time (t), g) by way of the derivative determined in step(f), checking the course of the internal tool pressure (Pwi) forpresence of a constant slope at least in regions of the internal toolpressure courses resulting from steps c) to d), h) when a non-constantdescending course occurs in step g), changing a holding pressure time(t_(n)) up to a transition between steps c) and d), i) repeating stepsa) to h) until a constantly descending course appears in step f). 11.The method according to claim 10, wherein step g) includes:selecting atleast one point from a plurality of points in regions of the course ofthe internal tool pressure resulting from steps c) and d), checking inregions upstream and downstream of each said point whether there is noconstant slope, in an absence of a constant slope, extending the holdingpressure time (t_(n)), measured as of a transition between steps b) andc), and repeating steps a) to g) until the checking results in aconstant slope.
 12. The method according to claim 10, wherein step g)includes:selecting a plurality of points in regions of the course of theinternal tool pressure (Pwi) resulting from steps c) and d), checking inregions upstream and downstream of each said point whether there is noconstant slope, in a presence of a constant slope in step g), shorteningthe holding pressure time (t_(n)), measured as of a transition betweensteps b) and c), repeating steps a) to g) until the checking does notresult in a constant slope, and performing steps h) and i) by slightlyextending the holding pressure time (t_(n)) until a constant slope isjust present.
 13. The method according to claim 10, wherein thedetecting step includes continuously detecting the course of theinternal tool pressure (Pwi) and step h) includes controlling theholding pressure time (t_(n)) in a closed control loop to a value sothat a constant ascending slope just still occurs.
 14. The methodaccording to claim 10, whereinthe checking according to step g) includesdifferentiating the course of the internal tool pressure (Pwi) once as afunction of the time to form a differential curve, determining a firstvalue (S1) of the differential curve in a region upstream of the holdingpressure time (t_(n)) and a second value (S2) of the differential curvein a region downstream of the point, and checking whether the firstvalue (S1) divided by the second value (S2) is approximately
 1. 15. Themethod according to claim 10, including using a digital signalrepresenting an end of the holding pressure as a starting point for thechecking according to step g).
 16. The method according to claim 10,wherein the machine is a hydraulic press and the method includes usingas a starting point for the checking according to step g) a point atwhich a hydraulic pressure of the press drops from a high level to a lowlevel.
 17. The method according to claim 14, wherein the holdingpressure time (t_(n)) is controlled according to a controller equationas follows:

    Δtn=-(S1/S2-desired value)×amplification factor.


18. A method according to claim 10, characterized in that the method iscarried out on a plastics injection molding machine on which steps a)and b) are velocity-controlled and c) and d) are pressure-controlled.